Organic EL elements can be divided into two groups of elements on the basis of process for making or property of material. The first elements are made of a material having low molecular weight using a deposition process, and the second elements are made of a material having high molecular weight using a coating process.
The first elements have disadvantages such as expensive production cost, and difficulty to apply for a large substrate or mass production, because it requires a vacuum deposition equipment. Meanwhile, the second elements have advantages such as cheap production cost, and simple manufacturing process, because it is easy to make film, composed of coating liquid, on a substrate, and removing solvent, which is included in the coating liquid.
When the thin film for an organic EL material is made by the coating method, the organic EL material is used to be dissolved into solvent. In this case, generally, the organic EL material having high molecular weight is dissolved into solvent. The solvent includes toluene, xylene, Tetralin®, mesitylene, and cyclohexylbenzene.
Meanwhile, when the organic EL material having low molecular weight is made by the coating method, wherein the organic EL material having low molecular weight is used to be dissolved into solvent, solubility of the organic EL material is important. The organic EL material having low molecular weight should have high solubility such as, for example 0.5 to 1.0% by mass, however most of the organic EL material having low molecular weight of prior art does not have high solubility.
For improving solubility, the method to introduce a soluble substituent, and method to decrease a symmetry of molecule are known (See also Japanese Patent Application Laid-Open (JP-A) No. 2008-166629). However when the soluble substituent is introduced, it is difficult to keep high thermal stability and temporal stability, because of its phase transition due to the soluble substituent such as alkyl groups. When the symmetry of the molecule is decreased, molecular design to ensure appropriate property is limited, because it requires asymmetric structure, bent structure, or bulky structure as a whole.
It is difficult to satisfy both of solubility and flexibility for molecular design.
Meanwhile, it is known that multiple films, wherein each of films has different function, are laminated in the organic EL element to increase its property. The deposition process is relatively suitable for laminating multiple films, because it is the vacuum process. The coating process is not suitable for laminating multiple films, because solvent of the upper film dissolves the lower film in laminating. When organic solvent and water solvent are used in wet coating process, two films can be laminated, but it is difficult to laminate more than three films.
To solve the problem in laminating, U.S. Pat. No. 4,761,006, JP-A No. 2004-505169, international publication No. 2008/038747, and international publication No. 2005/053056, propose high molecular compound having crosslinkable substituent, and also disclose method for laminating, wherein the compound becomes insoluble to organic solvent by its crosslink reaction. However, using this method smooth film is not obtained, or durability of element decreases, by a change of its volume associated with the crosslink reaction.
The method to satisfy all of solubility, flexibility for molecular design, and capability of laminating has not proposed.